Experimental evaluation of cobalt adsorption capacity of walnut shell by organic acid activation

Cobalt, from industrial waste and nuclear laundry, possess health risk to human beings, animals and plants. Number of methods, other than adsorption, have been reported in literature for Co removal from waste water. In this research walnut shell powder after modification has been utilized for Co adsorption. First step of modification involved chemical treatment by four different organic acids for 72 h. Samples were collected at 24, 48 and 72 h. Second step involved thermal treatment of 72 h samples. Unmodified and modified particles have been analyzed by chemical methods and instruments i.e. UV spectrometer, FTIR, cyclic voltammetry (CV) and microscopic imaging. Thermally treated samples have shown augmented Co adsorption. CV analysis showed thermally treated samples with better capacitance. Particles modified by oxalic acid presented better Co adsorption. Oxalic acid treated particles activated for 72 h with thermal treatment provided maximum adsorption capacity 1327 ± 20.6 mg/g against Co(II) at pH 7, stirring 200 rpm, initial concentration 20 ml, adsorbent dosage (5 mg) and contact time 240 min at room temperature.

www.nature.com/scientificreports/ as a tanning agent, and countless other applications of vegetables and fruits in the production of a vast array of consumer goods [30][31][32] . Such biowastes are particularly useful in a variety of applications, including adsorption, thanks to their structural characteristics and synthetic approaches 33 . Variety of aquatic pollutants including Pb 34 , As 35 , Hg 36 , rhodamine B 37 , methylene orange 38 , malachite green 39 , etc., could be removed through adsorption using facile and complex adsorbents 40,41 . Because of their high surface area and ease of functionalization, biowastes are incorporated into the production of a wide range of materials such as catalysts 42 , double layer capacitors 43 , polymers 44 nanoparticles 45 , supercapacitors 46 , etc. Different biowastes have been modified chemically or thermally to remove heavy metals including rice husk 47 , banana peel 48 , papaya peel 49 , kernel shell 50 , prawn shell 51 , bamboo bark 52 and many more. Agricultural biowaste such as apricot stone 53 , hazelnut shell 54 , buckwheat hulls 55 , lemon peel 56 , orange peel 57 , potato peel 58 , rice husk 59 , have been chemically or thermally treated to improve surface chemistry and textural characteristics that are ideal for co-adsorption. Because of its exceptional textural characteristics, walnut shell has been reported for high surface catalyst and nanoparticles 60 . This work presents a simple, novel, and eco-friendly method for producing metal adsorbents from walnut shell by treating it with four different organic acids and/or heat. The resulting adsorbents for Co(II) removal are effective, inexpensive, and benign to the environment.
Sample preparation. Walnut shell, after cleaning, has been crushed into powder (WP), sample preparation has been done as per data given in

Characterization
Prepared samples have been characterized by below-mentioned techniques FTIR spectra of prepared samples have been recorded using FTIR-ATR (IR-Spirit Shimadzu, Japan) within the range of 4000-500 cm −1 .
Micrographic pictures has been taken using "IRMECO GmbH & CO. Model No. IM-910" on calibration slide (0.01-0.1 mm) with 8X resolution to evaluate average size of agglomerates.pH is the negative log of concentration of hydrogen ions. pH is an important factor which has the ability to alter the obtained results of a reaction therefore it is important to keep tracking pH of prepared samples. Prepared samples (0.005 g each) in 10 ml of distilled water stirred at 450 rpm for 1 h, have been subjected to pH tester "Milwaukee pro waterproof pH/temp", and conductivity by using "JENWAY 4510 conductivity meter". www.nature.com/scientificreports/ Dispersion stability of prepared samples (0.005 g) has been tested in distilled water (10 mL). After 1 h constant stirring, time has been noted for samples to completely settle down 61 . CV of WP and prepared samples have been performed using "Gamry Reference 3000" with potential window (− 0.6 to + 0.6) under scan rate of 10 mV/sec in 1 M KOH.
Adsorption analysis. Adsorption analysis has been done as per method reported in literature 62 , with a slight modification of using 10 ml of 3000 ppm Co solution and 5 mg adsorbent, sampling at 15 min' interval, and analyzed by using UV-vis spectrophotometer (CECIL, CE74000) at λ max 625 nm. Percentage removal (Eq. 1) and adsorption capacity (Eq. 2) have been calculated by the following equations.
Here, C i and C t are the initial and final concentrations of adsorbate whereas M is mass of adsorbent in grams and V is volume of metal solution in liters used for adsorption analysis. Co(II) concentration (mg/L) calculations was done by Eqs. (3) and (4), adopted from a reported method 62 .
whereas "c" is concentration in mg/L, A is absorbance, "n" is volume (ml) taken for analysis. Acetic acid (AA) activated samples. FTIR absorbance spectra ( Fig. 1) of modified samples presented declining peaks at 2928 cm −1 (sp 3 C-H str), 1589 cm −1 (C=O sr) and 1031 cm −1 (alcoholic C-O str), signposting partial utilization of these functionalities at room temperature (physical adsorption of AA on WP) and complete utilization at high temperature i.e. sample AA4 (augmented reaction kinetics). Reduction of 1031 cm −1 peak suggests condensation, resulting in ester formation (peak at 871 cm −1 ), counter confirmed by the appearance of new peak at 1430 cm −1 representing C-H asymmetric bending of AA methyl group.    (Table 2). It has been observed that increased activation time augmented agglomerate size as well as conductivity, which may be attributed to increased functionalities and subsequently increased attractive forces upon samples' surface. However, thermal treatment has resulted in reduced agglomerate size, which may   Physico-chemical analysis. pH and conductivity results are given in Table 2. OA-1 has shown the lowest pH (2.7) and BA-4 the highest (7.3). It has been observed that activation time as well as thermal treatment augmented pH value. Conductivity trend have been found similar among prepared samples, but always higher than WP.

Results and discussion
Dispersion test. Dispersion analysis of unmodified and modified samples (Fig. 6) showed a quick settling of prepared samples (approx. 2 min) in comparison to untreated material (25 min). Fast settling of modified material showed hydrophobicity induced with surface chemistry change (see pH and FTIR), as well as increased agglomerate size due to condensation of surface functionalities incorporated on particle surfaces after modification.
Cyclic voltammetry. Cyclic voltammetry (CV) has been done to explain possible adsorption mechanism of modified samples (Fig. 7a-d). It has been observed that thermal treatment has intensified reduction peak allowing material to create stable interaction with metal cations. As per Fig. 7a, AA4 has highest surface conductivity evident from the highest peak, which marks it applicable for metal cations interaction. On the contrary, AA2 has shown the least conductance making its surface less interactive with metal cations. BA activated samples (Fig. 7b) have also shown similar pattern i.e. thermal treatment augmented conductive response than chemically modified samples. In OA activated samples (Fig. 7c) consistency in charge carrying capacity and activation time has been observed, yet still thermal treatment has developed reduction peak indicating material suitability for reducing metals. In SA activated samples (Fig. 7d), SA3 showed highest reduction peak which may be attributed to additional surface functionalities, as presented in FTIR (Fig. 4). Charge discharge behavior for unmodified and modified samples have been analyzed, through comparison of cyclic voltammograms taken at different time intervals (Fig. 8), and no change in current carrying capacity of unmodified and modified samples has been observed, verifying reusability of material. Adsorption analysis. Adsorption capacity of prepared samples against Co 2+ has been tested. Prepared samples have been dipped in metal solution for a pre-determined time with constant stirring. Metal solutions have been tested using UV-Vis spectrophotometer after specific time interval at wavelength of 625 nm.
UV analysis of prepared cobalt solution and its calculations have been performed by following the method already reported in literature 62 . UV-vis experimentation has been carried out at scan speed 1 nm/sec and scale at 25 nm/cm. All the samples have been analyzed between 400 and 1100 nm wavelength range (Fig. 9). Data showed a reciprocal relation between contact time and absorbance, indicating Co-adsorption. Furthermore, OA4 has been tested at different pH from 3 to 7 (Fig. 17) and found to have incremental effect on adsorption with www.nature.com/scientificreports/ high pH. Experiment designed in such a way that values have been obtained from each data set with replication number of 3 whereas accuracy determined to be the closest to expected results.
FTIR after adsorption. FTIR spectra of unmodified and modified samples have been taken after adsorption and found to have vibrations in 700-400 cm −1 range due to metal-oxygen bond 63 . AA activated samples have shown significant interaction against Co(II) ions (Fig. 10). Disappearance of peak at 1430 cm −1 (COO − str) in all variants of AA activated samples indicated Co(II) ions interaction with carbonyl functionalities present on sample surface which could be counter confirmed through enhanced intensity of peaks at 672 cm −1 and 666 cm −1 , attributed to metal-O bond formation 63,64 . All AA activated samples, after adsorption, have shown peak shifting form 1606 cm −1 (Carboxylic; COO − str) to 1505-1595 cm −1 , confirming metal chelation with carbonyl functionalities 65,66 . From the data it could be evaluated that proton shifting and ion exchange process helped in adsorption of cobalt ions on AA samples' surface.
SA samples (Fig. 10) consumed peaks at 1600-1200 cm −1 except a peak shift on 1515 cm −1 confirming carbonyl cobalt chelation. SA1 and SA3 showed peak reduction at 3220-3350 cm −1 (Carboxylic associated O-H str) after cobalt adsorption indicating consumption of OH group of salicylic acid present on the ring. Thermally treated sample after cobalt adsorption gave high intensity peaks at 1589 (aromatic; asymmetric COO − str) and 1378 (phenolic O-M str) indicating metal interaction with carbonyl and hydroxyl group.
WP showed involvement of aromatic moieties along with hydroxyl and carbonyl groups in Co adsorption (Fig. 11). Peak shifted from 1589 to 1515 cm −1 gave promising evidence regarding cobalt chelation with aromatic compounds on WP surface. Peaks shifted from 1242 to 1264 cm −1 and from 1344 to 1338 cm −1 respectively counter verified involvement of carbonyl functionalities. Peak shifted from 1036 (formats; CO-O) to 1031 cm −1 along with altered peak at 672 cm −1 confirmed cobalt and oxygen bonding.
Surface interaction and adsorption mechanism of prepared samples against Co(II) metal through ion exchange process has been supposed to be as Fig. 12. www.nature.com/scientificreports/ Adsorption capacities. Prepared samples showed significant results against cobalt adsorption capacities ( Table 3, Figs. 13 and 14) particularly in first 15 min of contact time, and later on adsorption process became slow due to less availability of active sites for Co adsorption. It has been observed that thermally treated samples except SA4 (Fig. 13d) showed maximum Co adsorption in comparison to other samples of same batch. OA4 showed maximum (1371.4 ± 20.6 mg/g) and BA2 (809.9 ± 21.1 mg/g) showed minimum adsorption capacity. Activation time played crucial role in adsorption behavior of samples. Samples with 24 h of activation (Fig. 14) have shown good adsorption against cobalt but 48 h activation have shown low adsorption which may be attributed to consumption of its surface functionalities i.e. dimerization of organic acids. Further activation (72 h) showed better adsorption among all chemically modified samples, whereas thermally treated samples showed the best adsorption, which may be attributed to thermal oxidation reactions on the surface of particles. Eventually, both surface chemistry and texture appear major. Optimum activation time has been found to be 72 h whereas thermal treatment enhanced samples' surface area and oxygen content. Modifiers, effect surface chemistry and adsorption capacity of particles (Fig. 14). AA, BA, OA and SA despite of similar active functionality (-COOH) have differ in chemical structure. www.nature.com/scientificreports/ Removal efficiencies. Removal efficiencies of modified samples in comparison of unmodified particle as per modification time (Fig. 15) and nature of modifier (Fig. 16) counter confirmed adsorption capacities of samples (Table 3). Maximum cobalt has been removed by OA4 (86.40 ± 1.3%) whereas minimum adsorption obtained from BA2 (50.04 ± 1.3%).   www.nature.com/scientificreports/ pH effect. OA4, as for its best output, has been tested out in the range of pH from 3 to 7, above this range precipitates of Co(II) ions formed as Co(OH) 2 69 , and increase in adsorption with pH has been observed. Initial concentration (10 ml), adsorbent dosage (5 mg), stirring (200 rpm) and contact time(1 h) taken as predetermined conditions to tested out pH effect on adsorption behavior of prepared sample. It could be seen (Fig. 17, Table 4) that minimum adsorption obtained at pH 3 whereas maximum adsorption obtained at pH 7. Adsorption increasing trend (Fig. 17) seems to be supported as pH increases which is also in agreement of literature 53 . www.nature.com/scientificreports/ Other samples have also been tested in different pH environment, as pH increased from 4 to 6, some samples have shown an increment in adsorption capacity including AA2 (911.7 ± 26.4 mg/g), AA3 (1221.6 ± 23.1 mg/g), OA2 (1386.94 ± 21.4 mg/g), SA3 (1304.67 ± 21.5 mg/g).

Conclusion
This research was aimed to prepare metal adsorbent from biowaste through facile chemical treatment, for effective an economical way of pollution control. Walnut shell powder has been treated with different organic acids at room temperature as well as at 550 °C. Thermal treatment of samples has shown maximum adsorption, same has been confirmed through CV analysis. The reason could be the development of magnetic behavior along www.nature.com/scientificreports/ with physical adsorption. OA4 sample has shown maximum adsorption (1371.4 ± 20.6 mg/g) and removal % (86.40 ± 1.3), signposting effective modification through OA. Analysis regarding effect of pH on adsorption capacity and % removal has shown incremental trend till pH 7. Results proved that biobased resources with suitable surface modification could be an economical, effective and efficient precursor for metal adsorbents in waste management applications.   www.nature.com/scientificreports/

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.